Method of manufacturing a capillary tube



Nov. 22, 1966 J. J, HRONAS 3,286,565

METHOD OF MANUFACTURING A CAPILLARY TUBE Filed June 17, 1964 5Sheets-Sheet 1 -//VPU 7' 2/ 24 25 INVENTOR. JOHN J. f/QO/VAS Nov. 22,

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"HAG/1 'WAGATQEAT 5 /0 x5 20 25 a0 a5 4 LENGTH (Fr) INVENTOR. JOHN J.f/ROA/AS United States Patent 3,286,565 METHOD OF MANUFACTURING ACAPILLARY TUBE John J. Hronas, Greentree, Pa., assignor to CalgonCorporation, Pittsburgh, Pa. Filed June 17, 1964, Ser. No. 375,891 2Claims. (Cl. 83--54) This application is a continuation-in-part of mycopending application Serial No. 215,701, filed August 8, 1962, now Pat.No. 3,212,677.

This invention relates to feeders or dispensers of liquid in smallvolumes at slow rates and over long periods of time.

Prior to the present invention, the continuous feeding of liquids atvery slow rates involved the use of elaborate and expensive equipment.Timers and/or pumps, for example, must be specially designed to achievethe slow rates usable for feeding corrosion inhibitors, biocides, sc-aleinhibitors and the like into industrial and commercial water systems.Indeed, a pump which is free of the hazards of breakdown, which does notrequire electricity or other power, which can be operated with virtuallyno supervision, which is of really simple design and yet which willdeliver only a few milliliters per minute or perhaps milliliters a dayat an even rate is difiicult to conceive.

I have invented a device which will deliver liquids at extremely slowfeed rates with virtually no complications since it involves no movingparts. My invention utilizes a capillary tube of extraordinary length.There I The presently preferred version will be discussed with 1reference to the accompanying drawings. FIGURE 1 is a more or lessdiagrammatic illustration of the presently preferred system;

FIGURE 2 is an illustration of a method of calibrating capillary tubeswhich simplifies manufacture of my invention;

FIGURE 3 is a graph showing the relation between air pressure drop in alength of capillary tube and its ability to dispense liquid at an evenrate;

FIGURE 3a is a graph of the same factors in a different range;

FIGURE 4 is a graph showing the effect on feed rates of internaldiameter and length of tubing; and

FIGURE 5 is an illustration of a tube as it could be utilized in avariation of my method invention for siphoning at very slow rates.

Referring to FIGURE 1, a plastic container 2 containing a liquid 1 to bedispensed is punctured near the bottom thereof by a metal probe 3 havinga bore 8 therethrough and a relatively sharp tip 4. In the exterior endof probe 3 is inserted a polyethylene capillary tube 5, which iscontinuous and which may be formed into a coil 6. If no siphoning actionis used, the full length of tube 7 is preferably kept below the level ofliquid 1.

Closure 9 of container 2 is preferably left slightly open or a hole 10is punched in the top of container 2 to permit air to enter to replacedispensed liquid.

The capillary tube 5 should be at least two feet long and preferablywill have an average internal diameter of about 0.01 inch to about 0.062inch, but may be up to about 0.2 inch and as little as 0.0 1 inch ID.The preferred tubing is made of polyethylene and may be purchased in3000 ft. rolls, and has an internal diameter of (1022":0001" and anouter diameter of 0.114". Very slow feed rates can be obtained by theuse of very long capillary tubes, for example on the order of feet.Conversely, more rapid flow rates can be obtained with lengths as littleas 2 to 4 feet. The diameter of the coil 6 also affects the flow rate.Generally speaking, the smaller the diameter of the coil, the lower theflow rate.

I employ a unique method of determining the exact length of capillarytubing needed to achieve a desired feed rate of a given liquid. Mymethod involves the use of air pressure and means for measuring the airpressure drop in a capillary tube. As can be seen in FIGURE 2, acapillary tube 29 slightly longer than will presumably possess thedesired delivery rate is attached at 20 and 21 to pressure gauges 22 and23 respectively. Beyond gauge 23 is a short length of tubing 24 havingmeans for receiving an input pressure at 25. The input pressure in thiscase is 29.75 as indicated on gauge 23. On the downstream end, a lengthof capillary tube 27 is attached at 26 to the gauge and the main line29. The end 28 of tube 27 is left open. To test the ability of capillarytube 29 to feed a particular solution or other liquid at a desired ratefor a known head (vertical distance from the liquid source level to thelower end), a known air pressure is introduced at point 25 and a readingis taken on gauge 23. Another reading is then taken on gauge 22, whileend 28 of tube 27 is permitted to remain open. The pressure drop(dilference between the readings of gauges 23 and 22) is then comparedto the air pressure drop previously taken on a length of tube known todeliver at the desired rate. A proportionate length of tube 29 is thencut off, and the remaining major length of tube 29 is kept for usewithout the necessity for further testing.

The air pressure drop may be used as a calibration and/ orstandardization reference for calculating feed rates of fluids ofvarious viscosities and under various pressure differences. The head, orheight of the column of liquid, and the force of gravity may beconsidered equivalent to a pressure difference throughout thisspecification. My invention contemplates the use of capillaries forregulating the flow of liquid where artificially-created pressuredifferences are the cause of flow, including very great pressuredifferences. It is not necessary to know the length of a tube noreventhe diameter to obtain a standard reference for flow rate. To obtain thedesired flow rate, the proper pressure diiference is used on a tube ofknown air pressure drop.

I prefer to perform the air pressure drop calibration on capillary tubesformed into a more or less permanent coil having a foot or two (ifpossible) of free end on each end. In this manner, variations due tocoiling are eliminated. I prefer to use coils of about 3 diametersecured roughly in toroidal form by common adhesive tape.

That the air pressure drop as calculated above is related to the feedrate is shown by the graph in FIGURE 3. To arrive at the data plotted inFIGURE 3, various lengths of tubing of approximately the same internaldiameter were used. They were formed into coils of about 3" diameter. Apressure of 29.7 p.s.i.g. was introduced into one end and anotherpressure reading taken at the other end. The downstream pressure readingis plotted on the X-axis and milliliters per day of liquid delivered bythe tube still coiled in the same manner through a 36" head is plottedon the logarithmic Y-axis. It will be noted that the two liquids used,tap water and a 50% aqueous solution of the corrosion inhibitor CALTREAT 214, present linear patterns on the logarithmic scale,

showing a clear mathematical relationship between the air pressure dropand the number of milliliters delivered per day for any given head. Thisrelationship holds true for any given tolerance of internal diameter;that is, it is a function of the integral of the internal volume of thetube.

An expanded range of data on the same factors obtained in the samemanner is shown in FIGURE 3a.

Table I lists downstream end pressure readings for various lengths of.022" ID. tubing having a manufacturers tolerance of 0.001". Asexplained above, a pressure of 29.7 p.s.i.g. was introduced to theupstream end. It is apparent that the feed rate of a tube of givenlength is predictable. It is also apparent that my invention operatesindependently of so-called capillary action. That is, my invention doesnot rely on the ability of a capillary tube to cause liquid to risewithin it.

TABLE I Downstream end Length: pressure reading 19.5 13.6

It will be seen from FIGURE 4 that the feed rate of tap water is readilycorrelated to the internal diameter and length of tubing. In FIGURE 4,the feed rates of tap water and a 50% solution of a corrosion inhibitor,Hagatreat 168 in a system similar to that illustrated in FIGURE 1, inwhich the vertical distance from the highest point of the tube 3 to itslower end (fixed head) was 36 inches, is plotted on the left side of thelogarithmic scale and the various lengths are plotted on the X-axis.Internal diameters (I.D.) and the particular liquids used in each testare indicated on the plotted lines. The curves continue asymptoticallytowards a flow rate of zero for longer tube lengths. A 100' length of0.22 (I.D.) tubing will deliver 20 ml. per day of Water under the sameconditions as in FIGURE 4.

My invention may be embodied in various ways. For example, the capillarytube may be pre-inserted through the bottle wall without the necessityfor the metal probe. That is, the bottle may be manufactured with anorifice or other device suitable to accommodate the capillary tube andestablish communication with the interior. The tube should be cappedwhen a filled container is sold with the tube already inserted, unlessthe tube end can be held higher than the liquid level. The feeder may besold, for example, with the tube already inserted and the coil of thetube encircling the neck of the bottle. Another variations is the tubeshown in FIGURE 5. In FIG- URE 5, a capillary tube 30 is shown ready forshipment. The tube 30 includes a coil 33. It is filled with the solutionit will be feeding in operation. The solution is held in the tube bycaps 31 and 32 at each end. Plastic clip 34 is placed on tube 30 aboutthe same distance from end 31 of the tube as the height of the bottlefor which it is intended. An optional weight 35 is attached near end 31.To place this variation in operation, the user simply removes cap 31,places the now open end in the solution container, clips the tube inplace by using clip 34, preferably on the lip of the container opening,allows the coil to dangle below the container, and removes cap 32 on thefar end of the tube. The device will now siphon the solution at apredetermined rate depending on the distance from the liquid level inthe container to the lower or free end of the tube. The clip is optionaland may be attached separately. It is not necessary to puncture thecontainer. This variation has the additional advantage that thecontainer closure will not inadvertently be left airtight.

The liquid may also be siphoned from the bottle with a capillary tubewithout the necessity of using a prefilled capillary tube as in FIGURE5. This method of dispensing liquids at even rates constitutes a methodwithin the scope of the present invention. The method may be describedas a method of dispensing liquid from a container thereof comprisinginserting an end of a rigid or flexible capillary tube at least threefeet long into the liquid through the top of said container, loweringthe other end of the tube below the surface of said liquid, and causinga flow of liquid to begin throughout the length of said tubing.

I have also found that a capillary tube may be standardized by applyinga known constant pressure input at 25 (FIGURE 2) and determining thepressure at gauge 23 for a previously standardized length of capillary29 open at the end remote from gauge 23 and without the use of gauge 22and tubing 27 and thereafter adjusting the length of an unknowncapillary to produce the same pressure reading at gauge 23 as wasobtained with the standardized capillary. This technique is, of course,simply another method of setting up a standard pressure drop across thelength of the capillary equivalent in end result to the practiceillustrated in FIGURE 2.

So far as I am aware, there is no maximum length of tubing which may beused in my invention, although practical uses for extremely slow feedrates are rare.

I do not intend to be restricted to the specific examples andembodiments with which my invention is explained herein. It may beotherwise variously practiced and embodied within the scope of thefollowing claims.

I claim:

1. Method of manufacturing a capillary tube of a known flow ratecomprising obtaining a capillary tube longer than the estimated lengthpossessing the desired flow rate, measuring the air pressure droptherein, and cutting off a length of tube proportional to the excessover the air pressure drop of a length of tube having the desired flowrate,

2. Method of manufacturing a capillary feeder capable of feeding at aknown flow rate comprising the steps of preparing a standard capillarytube of known flow rate, determining the air pressure drop from aconstant pressure source of air through said standard capillary andthereafter adjusting the lengths of desired additional capillaries toproduce a like air pressure drop from said constant pressure source ineach such capillary.

References Cited by the Examiner UNITED STATES PATENTS 2,075,921 4/ 1937Winkler et al 73-3 X 2,434,118 1/1948 Newman 62l27 2,879,140 3/1959Karasek et al 733 X 2,959,954 11/1960 Roberts 733 OTHER REFERENCESMarks, L. 8.: Mechanical Engineers Handbook, 4th edition, McGraw-HillBook Co., Inc., p. 264 (copy in Gp. 341).

WILLIAM W. DYER, JR., Primary Examiner.

F. T. YOST, J. M. MEISTER, Assistant Examiners.

1. METHOD OF MANUFACTURING A CAPILLARY TUBE OF A KNOWN FLOW RATE COMPRISING OBTAINING A CAPILLARY TUBE LONGER THAN THE ESTIMATED LENGTH POSSESSING THE DESIRED FLOW RATE, MEASURING THE AIR PRESSURE DROP THEREIN, AND CUTTING OFF A LENGTH OF TUBE PROPORTIONAL TO THE EXCESS OVER THE AIR PRESSURE DROP OF A LENGTH OF TUBE HAVING THE DESIRED FLOW RATE. 